This invention relates to an electrooptic modulator and more particularly to an electrooptic modulator of a diffraction type utilizing an electrooptic thin-film waveguide.
As coherenct light is incident in the medium wherein the propagation speed of the light wave is made periodically varied spatially in the direction perpendicular to the propagating direction of the light wave, the light beam is caused to be diffracted into orders emerging in separate directions. The intensities of the respective emergent beams are responsive to the amount of the spatial speed variation which originates from the modulation signal voltage applied to the interdigital electrodes provided on the surface of the electrooptic medium. In such a diffraction phenomenon the diffraction angles of the respective emergent beams are inversely proportional to the pitch of the spatially periodic electric field, and therefore the pitch should preferably be as small as a fraction of a millimeter to obtain a practically sufficient separation of the respective orders of emergent beams. But as it is known that such a periodic electric field concentrates within a depth of 1/2.pi. times the pitch, the region where the periodic electric field substantially exists consequently becomes very thin. Therefore in this type of an electrooptic modulator, the sufficient variations of the respective light beam intensities for the actually practical light modulator requires the light beam to propagate fully through the very thin portion where the electric field is concentrated.